Specific radio frequency bands are set aside for wireless cellular communications by the spectrum regulatory authorities (such as the Federal Communications Commission in the United States of America) to ensure the reliable operation of cellular systems, and are referred to as cellular bands. The term spectrum is commonly used to refer to the aggregate bands that are assigned to the cellular communication network in any given jurisdiction. Another analogous phrase to spectrum is radio frequency resources.
Cellular bands can be contiguous or non-contiguous and are typically divided into sub-bands, which again can be contiguous or non-contiguous, that are licensed to mobile network operators. A mobile network operator thus deploys the network infrastructure of a cellular system upon obtaining a spectrum utilization license, i.e. a license to use a particular cellular band or sub-band. Connection of User Equipment (UE) to system Access Points (AP) or Radio Units (RU) is then facilitated by a wireless radio air interface, referred to as the Radio Access Technologies (RAT), which utilizes a specific amount of spectrum. Such a RAT specifies the sets of signals, in terms of modulation, coding, multiple access, and all associated protocols required to achieve communication over the electromagnetic wireless transmission medium. Each AP could support one or more RAT and generally assigns bands or sub-bands to each of the RATs based on some manual pre-configuration that is usually based on a historical and/or projected usage for each RAT; RATs with more usage would be assigned more spectrum, and vice versa.
RATs are characterized by the required transmission bandwidth, transmission frame duration, frequency reuse factor between APs, user multiple access scheme, modulation and coding configurations along with the transmission and reception protocols. Due to the limited amount of spectrum available for cellular systems, RATs are designed with the objective of enabling maximal spectrum reuse at all system APs/RUs while having the highest possible spectral efficiency.
Due to variations in the capabilities of UE, multiple RATs are typically co-deployed in cellular systems. This consequently requires the partitioning of spectrum between co-deployed RATs. In most implementations, each RAT is assigned one or more blocks of spectrum, the bandwidth for each RAT typically being determined at the time of deployment, or perhaps manually reconfigured from time to time, to reflect typical usage of those RATs in the network. Spectrum partitioning between co-deployed RATs is typically applied at the system level, i.e. all system APs/RUs have the same spectrum partitioning between co-deployed RATs.
To improve spectrum utilization in multi-RAT systems, spectrum assigned to RATs experiencing continuously declining traffic demand is gradually deducted and reassigned at the system level to RATs with continuously increasing traffic demand in a process referred to as spectrum refarming. Multiple realizations of system-level spectrum refarming, with varying implementation complexity have been introduced. However, system-level spectrum partitioning between co-deployed RATs limits the amount of spectrum that can be deducted from a RAT; as the system must meet the traffic demand of all RATs at all system APs/RUs in all times. Therefore, the gains from spectrum refarming are limited to the long-term phase-out of technologies with declining traffic demand.